Light signals are required for the induction and regulation of many developmental processes in plants. Genetic analysis demonstrates that light responses are not simply endpoints of linear signal transduction pathways, but are the result of the integration of signals from a variety of photoreceptor systems. The phytochromes are red/far-red light absorbing receptors that have been shown to be light-regulated serine/threonine kinases. The subcellular location of phytochromes is regulated by light, being cytoplasmic in the dark, and imported to discrete nuclear sites in the light. Genetic and molecular analyses have identified a number of proteins that act downstream from phytochromes, in both the cytoplasmic and nuclear compartments, however traditional genetic screens have not identified all the components of the regulatory circuitry. Moreover, many mechanistic details are simply not known. It is proposed to use molecular, biochemical and genetic strategies to identify and characterize signal transduction components that act early in phytochrome signal transduction pathways. The primary aims of these studies are (1) to determine the mechanism of cytosolic retention of phytochrome B by PKS1, a cytoplasmic phosphoprotein; (2) to identify proteins involved in phytochrome B localization to subnuclear pool; (3) to characterize new components in the PHY signaling pathways identified in genetic screens; and (4) to assess the level of protein trafficking in response to light. In summary, the proposed experiments will further our knowledge of phytochrome-mediated signal transduction pathways, and will address the mechanism by which phytochromes transmit light signals to regulate such diverse responses as changes in gene expression, cell expansion and division, and chloroplast development. The long-term objective is to completely understand the signal transduction networks that allow plants to perceive and respond to their local light environment, and to relate this to general mechanisms of signal transduction in other organisms.